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30V / 1A high-efficiency buck DC/DC converter with optional SSCG to reduce EMI emission

March 22, 2021

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R1271 is a new compact 1A synchronous buck DC/DC converter, operating up to 30V (Abs. Max. 42V) input voltage. The converter features a controller and integrated high and low side N-channel MOSFETs, providing a small, high-efficiency power supply with a range of additional safety circuits, intended for consumer, industrial and automotive applications (AEC-Q100 compliant soon).

DC/DC converters are increasingly popular to use in many kinds of applications, as they owe their popularity in terms of efficiency and extending battery life. However, the fast switching devices make them a prime potential EMI source. Therefore, the R1271 has a Spread-Spectrum Clock Generator (SSCG) for diffused oscillation frequency in PWM operation, which is available as an optional product version. The idea behind using SSCG is to modulate the clock frequency mildly, which results in spreading the EMI energy amongst many frequencies instead of concentrated at one frequency. In this way, the amount of potential interference of the base frequency and its harmonics is reduced considerably.

Effectiveness of SSCG (Spread Spectrum Clock Generator)

SSCG Enable vs SSCG Disable ex:R1275S: VOUT=5V, FOSC=2MHz

SSCG Enable vs SSCG Disable ex:R1275S: VOUT=5V, FOSC=2MHz

The R1271 has a special protection for cranking conditions. A power supply of automotive electronic systems can be subject to a significant input voltage drop when the engine starts. To ensure that the R1271 is able to maintain its output voltage during such event, the clock signal is linearly lowered to one-fourth of its normal frequency. This will make the on-duty larger than the normal maximum duty, which reduces the voltage difference between input and output and provides continuous operation in cold cranking condition.

Frequency modulation by duty over

Cranking wave form:VOUT=3.3V,ROUT=3.3Ω

The device has a series of protection circuits that contributes to the application’s safe operation. The Power-Good pin provides a fault signal to the system when the device is not ready for normal use or when one of the following protection circuits or states is active:

  • CE pin low, chip is disabled
  • UVLO or OVLO protection active, input voltage drops below 3.3 V or rises above 35 V.
  • Thermal Shutdown protection active, chip junction temperature rises above 160ºC
  • Soft-Start time, during start-up or restart
  • UVD & OVD active, output voltage deviates ±10% from its nominal setting
  • Hiccup or Latch type overcurrent protection active

As for the overcurrent protection, the designer has an option to select a product version with either Hiccup-mode or Latch mode protection, depending on the application’s requirements. Hiccup-mode overcurrent protection shuts down the chip after detecting an overcurrent condition and tries to resume to normal operation after a delay time. The Latch mode overcurrent protection will not return to normal operation automatically, but requires a reboot of the chip after removing the over-current condition.

The output current is limited by a peak current limit control method and the threshold is internally set but can be adjusted by connecting an external resistor to the LIMIT pin.

The soft-start time is the time interval between controlling the CE pin to active until the output voltage reaches the set output voltage, it assures a smooth ramp up of the output voltage. This timing is internally set to 500 µs, but can be extended by connecting an external capacitor to the CSS pin.

Phase compensation is done internally to reduce the number of external components, another measure to design a power supply with a small PCB space.

Recommended external components are a 2.2 µH inductor and two 10 µF input and output capacitors, the R1271 is available in a compact DFN3030-12B or regular HSOP-18 package (under development). The DFN3030-12B package has a special pin layout to prevent damage to the chip in the event of a short circuit between adjacent pins.

Conclusion

The R1271 is intended for use in a wide variety of applications that require a higher degree of reliability; we have product versions available for automotive, industrial, as well as consumer equipment. It has a range of safety measures, an optional SSCG circuit to reduce EMI emission and the choice to select from two different current limit circuits. Output voltage setting and phase compensation are done internally to reduce valuable board space; the DFN3030-12B package including external components requires only 69mm2.

This is especially important for safety-critical automotive applications such as ASIL-B / C systems including ECU, ADAS and control units, including EV converters and charge regulators. The R1271 is included in our product longevity program in order to support specific long-term projects; products included in this program remain available for at least 10 years.